Harmonic analyzer



March 9, 1937."

P. O. FARNHAM HARMONI C ANALYZER Filed Oct. 17, 1935 2 Sheets-Sheet 2 (A/VAZYZQ? 055/0 1) INVENTOR PAUL 0. FAfiW/MM BY 2 QWM ATTO R N EY Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE HARMONIC ANALYZER Paul 0. Farnham, Mountain Lakes, N. 1,. assignor to Radio Corporation of America, a corporation of Delaware My present invention relates to adjustable high frequency filter systems, and more particularly to a harmonic analyzer which utilizes variable resistor elements as the frequency selection means.

One of the primary objects of my invention is to provide a filter system for eliminating the fundamental frequency of an electrical wave, the filter system being essentially characterized by its freedom from inductive elements thereby avoiding magnetic pick-up, and its wide fundamental frequency range.

Another important ob-ject of the invention is to provide a compact, light electrical wave analyzer wherein inductive elements are omitted, an adjustable resistor-capacity network being employed to cancel out the fundamental of a wave whose harmonic content is to be tested.

Another object of my invention is to provide in an adjustable filter system, a multi-section resistor-condenser network, wherein the resistors or the condensers of the network are variable to adjust the filter over a wide range of fundamental frequencies, at least three sections being employed between the input and output terminals of the filter whereby a predetermined fraction of fundamental energy fed from the input of the filter to the output thereof will cancel out the energy of fundamental frequency at the output terminals.

Other objects of the invention are to improve generally the simplicity and efficiency of wave elimination filters, and more especially to provide filters which are not only simple and reliable in construction, but economically manufactured and assembled.

The novel features which I believe tobe characteristic of my invention are set forth in par- I ticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit or- 4 ganizations whereby my invention may be carried into effect.

In the drawings: 7

Fig. 1 shows a generalized embodiment of the 50 invention,

Fig. 2 shows an alternative terminating output circuit for the filter system of Fig. 1,

Fig. 3 shows a further modification of the output circuit, and the latter embodying a compensa- 5 tion network,

Fig. 4 shows a further modification of the filter system of Fig. '1,

Fig. 5 graphically shows a comparison of the performance of the analyzer circuits disclosed in Figs. 1 and 4.

Referring now to the drawings, wherein like reference characters'in the different figures designate similar circuit elements, there is shown in Fig. 1 a general method of eliminating the fundamental frequency of a complex wave form to be analyzed. The wave source W feeds electrical waves to the input of a multi-section filter network; the latter comprising series condensers C1-Cz-C3 and shunt resistors Ro-R1R2R3.

The resistors R1'R2R3 are designated as adjustable by the arrows therethrough; the dotted line I denoting that they are simultaneously varied in magnitude by any desired type of uni-control means. A lead 2, having an adjustable tap 3, is arranged to impress upon the output terminals of the filter network a predetermined fraction E'oOf the input voltage E0.

Without entering into theoretical considerations, it can be stated as readily demonstratable that for a given frequency theoutput voltage E will be of exactly opposite phase to that of the in'put'voltage E0, and at the same time will bear a definite amplitude ratio to En, depending on the number of sections of C and R in the network. It is further true of the network shown that the number of C R, sections employed must be" more than two to obtain phase opposition for E E", E'o, of the total input voltage at fundamental frequency may then be used to buck out, or cancel, the out-of-phase transmission E, and toobtain transmission, therefore, of harmonics only to a succeeding amplifier.

It can be shown by analysis that when C1=C2=C3=C, and R1=R2=R3=R the fractionto be greater. than zero. A certain fraction,

I a1 voltage needed for cancellation in Fig.

1 is Furthermore, the tuning of the networ kfto the fundamental frequency is accomplished by adjustingthe' resistors; or condensers.

to satisfy the relation 5, where X,

. R wC These relations are independent of the frequency to be eliminated, and indicate that the fractiona1 voltage need have only a minor additional adjustment of the order of 1% to obtain a perfect cancellation over a wide frequency range. It is also evident that the range over which the frequency may be eliminated is a linear function of 01 T2: case of tuning a coil. Thus, a ten to one range in eliminated frequency is attained with a ten to one range in either C 01' R, instead of a one hundred to one range of C as in the case of tuning a coil.

There are additional advantages in the general circuit arrangement of eliminating a fundamental frequency of a complex electrical wave. No inductances whatever are utilized in the network thus completely avoiding magnetic pick-up. Not only will such a network be compact and have a light weight, but it possesses a wide frequency range of fundamental which can be eliminated. Cancellation of the fundamental frequency is readily secured by a simple adjustment of the resistor magnitudes, or the condenser magnitudes if desired, and a proper adjustment of tap 3 on resistor R0. The output circuit of the filter network may assume various forms.

Thus, in Fig. 2 there is shown an electron discharge tube 5 having its input electrodes connected across the resistor R3. The control grid of tube 4 is connected to the junction of condenser C: and resistor R3 through a resistor 5, and the bucking voltage is applied to the control grid of tube 4 through a resistor B. The terminating arrangement shown in Fig. 2 sacrifices some sensitivity off balance (transmission of harmonics), but in general is suitable and reliable except for a constant factor due to transmission loss in the bridge network located at the input electrodes of tube 4. Resistors 5 and 6 should be made large compared with the maximum value of R3.

The terminating arrangement in Fig. 3 is preferable, and comprises a pair of tubes 1 and 8, the tube 8 having impressed on its control grid the bucking voltage E'o. The tube 1 has the output voltage E impressed between its input electrodes. The resultant output voltage of tubes 1 and 8, with the fundamental frequency eliminated, is derived across network 9 disposed in the common output circuit of tubes 1 and 8. The energizing direct current source of the tubes is not showrf in order to simplify the circuit diagram, and those skilled in the art will readily appreciate the manner of locating these current sources. The network 9 is designed to enable one to secure approximately constant transmission over all characteristics for a large number of hormonics.

It is not believed necessary for the purposes of this application to give the analysis used to derive the design of specific compensation networks, or the various harmonic transmission ratios. Each compensating network is designed to secure uniform transmission overall of the harmonics in combination with the net transmission characteristic of the filter network. In Fig. 3 is shown a general manner of locating a compensation network. Those skilled in the art will readily appreciate the manner of constructing and utilizing these compensation networks. It is sufficient to point out that the frequency response characteristic of the compensation network, for a proper overall transmission of frequencies from the second to the tenth harmonic, will be the inverse of the portion of characteristic A in Fig. 5 from abscissa 2 to It! for example. It may be noted that as the resistors of the filter 1 instead of being as 1n the usual networks are adjusted to eliminate the fundamental frequency, the compensating network should likewise be adjusted to track with it so as to operate equally at whatever fundamental frequency is being used. In other words the adjusting element of the compensating network may be conjointly controlled with the main adjustment device of the filter network.

The rate of adjustment can be readily calculated by anyone skilled in the art when the desired characteristics are known. The total harmonic voltage measured by this device is where E2, E3 etc. represent the amplitudes of the second, third, etc. harmonic voltages. A comparison between Eh and the fundamental voltage amplitude E1 (En/E1) is called the distortion factor of the wave, and is well known in the art.

In Fig. 4 there is shown an alternative embodiment of the filter network disclosed in Fig. 1. In this case the resistors are arranged in series in the network, and have the symbols R'1R2 B's, and the condensers are arranged in shunt and are denoted by the symbols C1C'2Ca. In this embodiment the fundamental bucking voltage Eo is again derived from across resistor R0, and a fractional voltage of is used. The harmonics go chiefly through the bucking path in this arrangement.

Fig. 5 graphically shows a comparison between the operation of the general filter network shown in Fig. 1 and that shown in Fig. 4. That is, the filter networks are compared with C and R interchanged in location. It is believed that the relations shown in Fig. 5 are self-evident. In either filter network design the bucking voltage E's, corresponding to Eo is equal to of the input voltage E3. It will be observed from. the graphical analysis of Fig. 5 that the filter network denoted by curve A gives the greater transmission for harmonic analysis, whereas the filter network of the typev whose characteristic is depicted by curve B gives the greater transmission in connection with sub-harmonic analy- SIS.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a source of electrical waves comprising a fundamental frequency and harmonics thereof, a multisection filter network including at least three filter sections, each section consisting of a resistor and a condenser, the magnitudes of the resistors and condensers of said sections being so chosen as to adjust the network to produce a phase shift of irN radians in the output of the said fundamental frequency, N being a whole number other than zero, and means for adding a fraction of the wave input voltage to the output of said filter network, said fraction having a magnitude such that voltage of fundamental frequency is eliminated in the total output of said filter network.

2. In a system as defined in claim 1, the condensers of the said sections being arranged in series with each other.

3. In a system as defined in claim 1, the resistors of said sections being arranged in shunt with each other.

4. In a system as defined in claim 1, means for conjointly adjusting the magnitudes of said resistors to vary said filter network over a desired fundamental frequency range.

5. In a system as defined in claim 1, a compensation network disposed in the output of said filter network for rendering uniform the overall transmission characteristic of the harmonics of the electrical waves.

7 PAUL O. FARNHAM. 

